[Illustration: FIG. 4.—_Astacus fluviatilis._—In A, the gills, exposed by the removal of the branchiostegite, are seen in their natural position; in B, the podobranchiæ (_see_ p. 75) are removed, and the anterior set of arthrobranchiæ turned downwards (× 2): _1_, eye-stalk; _2_, antennule; _3_, antenna; _4_, mandible; _6_, scaphognathite; _7_, first maxillipede, in B the epipodite, to which the line points, is partly removed; _8_, second maxillipede; _9_, third maxillipede; _10_, forceps; _14_, fourth ambulatory leg; _15_, first abdominal appendage; XV., first, and XVI., second abdominal somite; _arb. 8_, _arb. 9_, _arb. 13_, the posterior arthrobranchiæ of the second and third maxillipedes and of the third ambulatory leg; _arb′. 9_, _arb′. 13_, the anterior arthrobranchiæ of the third maxillipede and of the third ambulatory leg; _pbd. 8_, podobranchiæ of the second maxillipede; _pbd. 13_, that of the third ambulatory leg; _plb. 12_, _plb. 13_, the two rudimentary pleurobranchiæ; _plb. 14_, the functional pleurobranchia; _r_, rostrum.] Thus the branchial chamber is altogether outside the body, to which it stands in somewhat the same relation as the space between the flaps of a man’s coat and his waistcoat would do to the part of the body enclosed by the {27} waistcoat, if we suppose the lining of the flaps to be made in one piece with the sides of the waistcoat. Or a closer parallel still would be brought about, if the skin of a man’s back were loose enough to be pulled out, on each side, into two broad flaps covering the flanks. It will be observed that the branchial chamber is open behind, below, and in front; and, therefore, that the water in which the crayfish habitually lives has free ingress and egress. Thus the air dissolved in the water enables breathing to go on, just as it does in fishes. As is the case with many fishes, the crayfish breathes very well out of the water, if kept in a situation sufficiently cool and moist to prevent the gills from drying up; and thus there is no reason why, in cool and damp weather, the crayfish should not be able to live very well on land, at any rate among moist herbage, though whether our common crayfishes do make such terrestrial excursions is perhaps doubtful. We shall see, by-and-by, that there are some exotic crayfish which habitually live on land, and perish if they are long submerged in water. * * * * * With respect to the internal structure of the crayfish, there are some points which cannot escape notice, however rough the process of examination may be.